Radiolabelling as a method of diagnosis or treatment has been in use for many years. The continuing challenge has been to maximize concentration of the radioisotope in the area or region of interest, diseased tissue or tumor, while minimizing the concentration of the radioisotope in other areas and thereby minimizing damage to healthy tissues.
The paper by S. Ning, K. Trisler, D. M. Brown, N. Y. Yu, S. Kanekal, M. J. Lundsten, S. J. Knox: "Intratumoral radioimmunotherapy of a human colon cancer xenograft using a sustained-release gel", Radiotherapy and Oncology, 39, 179-189, 1996 discusses an intratumoral injectable gel drug delivery system for local administration of radioimmunotherapy. The injectable gel was a collagen-based drug delivery system designed for intratumoral administration. The study demonstrated that intratumoral delivery of radiolabeled antibodies using the collagen gel system markedly increased the retention of radioisotope in the tumors, enhanced the antitumor efficacy, and reduced the systemic toxicity compared to systemic administration of the radiolabeled antibody. Ning et al. teach the use of injectible collagen gels that are not stimuli-sensitive. Moreover, these collagen gels neither fully perfuse tumor tissue nor do they hold the radioisotope within the collagen gel matrix. Thus, the radioisotope is attached to an antibody for perfusing and binding to the tumor tissue. Lack of perfusion of the collagen gel and limited range of radioisotope decay products require that the radioisotope leave the collagen gel matrix to achieve close proximity with tumor tissue to achieve the therapeutic effect.
In the paper PRELIMINARY EXPERIENCE OF INFUSIONAL BRACHYTHERAPY USING COLLOIDAL .sup.32 P, S E Order, J A Siegel, R Principato, L S Zieger, E Johnson, P Lang, R Lustig C Kroprowski, P E Wallner, Annals Academy of Medicine, May 1996, Vol. 25, No. 3, an infusion by a needle into a tumor was done without the need for an arterial catheter and eliminating the need for hospitalization. This paper reports using dexamethasone (Decadron) to overcome intratumoral resistance followed by macroaggregated albumin then colloidal chromic phosphate .sup.32 P followed by more macroaggregated albumin injected into the tumor. Sufficient radiation emitted by the radioisotope leads to tumor cell killing and remission of solid cancers. However, disadvantages of this method include the serial injections and leakage of .sup.32 P from the tumor.
There is need in the art for a method of introducing a radioisotope into a localized area with a single or multiple injection(s) as well as a need for a local delivery system with little or reduced leakage of the radioisotope.
Stimulus-sensitive reversible hydrogels are herein defined as copolymer-solvent systems that undergo a transition between a solution and a gel state in response to the external stimuli such as temperature, pH, ionic strength, solvent composition, sheer stress or a combination of these factors. A reversible stimuli-sensitive gel is one in which the transition is reversed upon reversal of the stimulus. A well known example of a reversible hydrogel is an aqueous solution of gelatin that is in a solution state at high temperatures (e.g. 80.degree. C.) and forms a gel at lower temperatures (e.g., 20.degree. C.). Other examples of reversible gels involve aqueous solutions of agarose and kappa-carrageenan that gel in response to the temperature change, and aqueous solutions of alginate that gel in response to the increased concentration of calcium ions. Reversible hydrogel systems are used in food and pharmaceutical industries as thickeners and suspending agents.
Some specific reversible gelling copolymers were also investigated as drug delivery systems and tissue engineering polymer matrices. High viscosity aqueous solutions containing 20 (or more) wt % of block copolymers of polyethylene oxide and polypropylene oxide, e.g. Poloxamer 407 and Pluronic F68 (Poloxamer 188) exhibit reverse thermal gelation. Solutions of Poloxamer 407 have been investigated for intraocular administration. Solutions containing 25 and 30 wt % of Poloxamer 407 have been prepared and the force needed to inject them through a 25 GA needle was investigated. It was concluded that a liquid-gel transition occurred inside the needle, due to the heat transfer between the needle walls and the surroundings. [J. Juhasz, A. Cabana, A. Ait-Kadi, EVALUATION OF THE INJECTION FORCE OF POLOXAMER 407 GELS FOR INTRAOCULAR ADMINISTRATION, Pharm.Res., 13, No.9, 1996, Symposium Supplement, S-276].
In another example, 25 wt % aqueous solution of Pluronic F68 was mixed with articular chondrocyte cells suspension at 4.degree. C. and injected subcutaneously in nude and immunocompetent rabbit. In both cases, the cells entrapped in the copolymer formed tissue with histological appearance of hyaline cartilage. It was concluded that thermally reversible Pluronic F68 gel can serve as an effective injectable matrix for tissue engineering. [C. A. Vacanti, et al., Proceedings of Tissue Engineering Society, Orlando, Fla., 1996].
An example of a pH-reversible hydrogel, investigated as an in situ gelling system for ophthalmic use is the aqueous solution of, a poly(acrylic acid )polymer, which undergoes a pH-mediated phase transition at concentrations above 0.1 wt %. The solution also contains hydroxypropyl methylcellulose, a viscosity enhancing agent. [Pharm.Res., 13, No.9, 1996, Symposium Supplement].
A new vehicle for topical and mucosal delivery, based on reversible gelation, was developed as an interpenetrating polymer network (IPN) of poly(acrylic acid) and a block copolymer of poly(ethylene oxide)/poly(propylene oxide). When heated from ambient to body temperature the network exhibited a significant viscosity increase from a viscous liquid to a gel-like consistency. It was concluded that at higher temperature, reduced release rates of active ingredients from the network were observed due to the increased viscosity of the IPN. [E. S. Ron, et al., A NEW VEHICLE FOR TOPICAL AND MUCOSAL DRUG DELIVERY, Pharm.Res., 13, No.9, 1996, Symposium Supplement, S-299].
All gels containing the copolymers of poly(ethylene oxide)/poly(propylene oxide), i.e., Poloxamer 407, Pluronic F68 (Poloxamer 188), an IPN of poly(acrylic acid) and a block copolymer of poly(ethylene oxide)/poly(propylene oxide), and combinations thereof exhibit a limited, concentration dependent, stability of the gel state. The gels formed from these copolymers become liquids upon dilution (as for example due to the dilution with body fluids after peritoneal injection). Additionally, all the above examples of reversible hydrogels exhibit high initial viscosity in a liquid state, i.e., before the gelling transition.
Accordingly there is a need for a reversible gel that only reverses when a specific stimulus is reversed and does not reverse upon introduction of a different stimulus (e.g. dilution). Moreover, there is a need for a reversible gel that has a lower initial viscosity.
The U.S. Pat. No. 5,262,055 to Bae et al. discusses an artificial pancreas utilizing reversible gels based on NiPAAM and its copolymers. These polymers and copolymers do not reverse upon dilution and they have a lower initial viscosity. However, the NiPAAM homopolymer described in Example 1 of Bae et al. forms a dense gel with minimal water content (i.e. exhibits substantial syneresis).
Accordingly, there remains a need for a thermally reversible gel without substantial syneresis.
Polymers exhibiting phase transitions in water have many potential uses for drug delivery as stated in GRAFT COPOLYMERS THAT EXHIBIT TEMPERATURE-INDUCED PHASE TRANSITIONS OVER A WIDE RANGE OF pH, G. Chen, AS Hoffman, Nature, Vol 373, Jan. 5, 1995 (pp49-52). In this paper, the authors further describe a temperature sensitive polymer that phase separates with a change in temperature or pH. Chen and Hoffman use graft copolymers having side chains of a temperature sensitive homopolymer, the oligo-N-isopropylacrylamide, grafted onto a pH sensitive backbone homopolymer of acrylic acid. The authors describe the phase separation of the graft copolymer investigated by a cloud point determination in dilute solutions. However, a dilute solution cannot produce a reversible gelation of these graft copolymers. Chen and Hoffman also mention random copolymers of N-isopropylacrylamide and acrylic acid as exhibiting a phase separation, however, there is no description of the intention to study the possibility of reversible gelation in more concentrated solutions of these random copolymers.
Thus, there is a need for a stimulus sensitive gel with radioisotope that is useful in infusional brachytherapy.